My own personal theory on it is that it's really dilute mud, mixed with MSG and then heated to appreciable tounge scorching temperatures. Yes, even Starbucks is better than Seattle's Best, they switch the MSG for sugar, after all.

Seattle's Best is usually found in odd places around town, where people aren't trendy enough to drink Starbucks, yet not non-conformist enough to drink Tully's. It also worms its way into places where tourists are bound to be, like SeaTac, in the hope that they'll be suckered by the catchy title.